The patents to Bauersfeld, U.S. Pat. No. 1,943,509 and Bauersfeld et al., U.S. Pat. No. 1,943,510, disclose conventional vertical illumination systems which may be used for either brightfield or darkfield microscope. The same illumination axis is used for both brightfield and darkfield microscope although different portions of the path are utilized for different types of microscope. These illuminators do not employ axicons, i.e. a reflective conical surface, mirrors.
The patent to Hauser et al., U.S. Pat. No. 1,988,946 discloses an illumination system that employs two axicon mirrors, but it does not have a third mirror and it does not provide 100% of the available light for darkfield illumination. It is not easy with this system to switch efficiently or to balance light sources.
The patent to Grosser, U.S. Pat. No. 4,317,613, describes an illumination system that uses different torroidal reflecting surfaces always in combination with a transparent, conical device (axicon) which functions as a refracting optical element for low numerical aperture (NA) beams, and as a total internal reflecting *TIR) element for high NA beams. When simultaneously several modes of operation are provided, the illuminating beam is divided into several components, resulting in reduced power in each branch. Rapid switching between modes of operation cannot be accomplished at high rates (i.e. video rates) or under microprocessor control, but requires operator intervention for re-positioning components and adjustments.
The systems described above work sufficiently well for use by a skilled operator. The application of these methods to visual automated inspection tasks is limited due to several problems. One problem is that the switchover from one mode, for example, incident brightfield, to the other mode, incident darkfield requires human intervention which is not practical in computer controlled systems. Another problem is that the present systems present a highly magnified image to the human eye which has a relatively low resolution capability (10 lines/mm). In addition, the present systems make little effort to maintain constant light levels for different modes of observation because of the eyes immense dynamic range which is believed to extend over 12 to 15 decades. However, light detection systems, such as the vidicon tube or CCD array camera used in automated microscopes may have 5 to 10 times the resolution, but have a dynamic range of only three decades. As a result, in automated systems the light levels should be independently adjustable for efficient use in both the brightfield and darkfield observation modes. In fact, significantly more light is required for darkfield illumination because only scattered light is collected and directed to the photodetection system. Since present darkfield illumination systems are substrative, they typically use only 50 to 80% of the available light and significant differences occur between brightfield and darkfield illumination levels.